The salmon-colored Moderne-style Fox Plaza, extending 34 stories above the 20th Century Fox lot at Avenue of the Stars and Olympic Boulevard, has been sold to LaSalle Partners Ltd., a commercial real estate services firm headquartered in Chicago.

The seller is Studio Properties, an affiliate of 20th Century Fox, according to Jim Gallagher, a vice president in the Los Angeles office of LaSalle Advisers, a subsidiary that manages pension funds and endowments.

The size of the deal, which remains in escrow, was not disclosed. Published reports have put the price in the neighborhood of $320 million. If so, at $450 a square foot, Fox Plaza would be Los Angeles' most costly piece of commercial real estate.

The tower occupies six acres of 20th Century Fox's 63-acre lot. Its top floor is leased by the President Reagan Foundation, and Reagan is expected to occupy office space there on leaving office next January.

Building Fully Leased

Thanks to the distinctive prism-like faceting of its upper stories, the tower provides up to 16 prized corner offices on a floor, each offering a sweeping view of the Los Angeles basin, the Pacific Ocean or the Santa Monica Mountains. The 711,115-square-foot structure built on a strategic crest in Century City, was designed by R. Scott Johnson of Johnson Fain & Pereira Associates (an architectural firm known until last June as Pereira Associates).

The building, which opened for occupancy in January, 1987, is fully leased.

"We are going to hold it for a long time," Gallagher said Wednesday. "We invest for domestic pension funds and endowments, and Fox Plaza is the type of product we are looking for--high-quality, well-leased, well-located commercial properties. It certainly makes the grade on all counts."

Management arrangements have not been decided, he said, but LaSalle has its own management firm, LaSalle Partners Management. The property is managed by Tishman West.

LaSalle Partners also owns an office building at 1990 N. Bundy Drive in West Los Angeles, the Industrial Complex of Camarillo in Ventura County, the Union Bank Building at Oceangate in Long Beach and the former Crocker Plaza building in that city.